Speed Bot/Dragster

So my class is trying to build a dragster or a speed bot. We are only allowed 2 motors. What is the best gear ratio for this scenario (we are running this in classroom floors)? What wheel layout should I use? How many wheels should I use? Thanks.

So are there any turns you have to make or is it straight line?

1 Like

if all you have to go is a straight line, then you can probably go quite fast with 2 motors. I’m going to assume the motors are v5, because it will make things much easier and faster.

you’re going to want to use omni wheels probably, because they will grip classroom floors much better than traction wheels. 4" omni wheels will be best, the larger the wheel the faster it goes. if you have 600 rpm motor cartridges available, that could be a good option. you wouldn’t need any bulky gear ratios, and 600rpm 4" wheels would move your robot at roughly 10 feet per second. that’s pretty fast.

you’ll want to build as light as possible for the sake of acceleration. I’d recommend building as small as possible. use aluminum for the chassis. I’d also recommend using 3 wheels, 2 wheels powered by your motors in the front of the robot, and one free spinning wheel in the back. it doesn’t matter how big or what type this wheel is. if you can build it well enough, it’s possible you’ll be able to make it even faster. perhaps 200 rpm motors on a 1:5 gear ratio. this would move at 17 feet per second. but it would probably have very slow acceleration, and depending on the length of your ride may mean 600 rpm would give you better time.

good luck!


Just a straight line of 20 feet

You are right and thanks so much! This will really help me!

There is no single “best gear ratio.” The beat ratio will be affected by all of the following:

  • weight of the bot
  • length of the race (do you have time to make up a slow start?)
  • diameter of the wheel
  • strength of the motor

I suggest trying to design such that you can easily change ratios and test (and test, and test).

Lower ratios will favor your initial take off. Higher ones will give you more top speed. At 20 feet your likely going to want to optimize for start speed. And if possible don’t discount having the bot react to the go signal. It can likely react much faster than a human.


If you want to make the robot really fast, you can get yourself virtually infinite torque with rubber bands, and impress everyone with only using one motor to merely release the mechanism.


When you think about it, anything can be broken with an infinite number of rubber bands.


I bet you can break records with enough rubber bands




You gotta love rubber bands; so useful for so many applications.


make a differential swerve so that you can maximize both your acceleration and the max speed by changing the MA

1 Like

Why would you need to switch which way the wheels is facing. Also I’m assuming you mean F=MA. You would either need to switch gear ratios via planetary or by using a manual like transmission.

no. MA as in mechanical advantage. changing the angle of the wheels on a swerve drive changes the mechanical advantage, just like how the MA of a tank drive is different than the MA of a traditional x drive of the same internal and external gearing and same wheel diameter, due to how the wheels are angled


But you need 2 motors to move turn one wheels so you would need to link them together somehow. Also considering you would only get sqrt2 times faster compared to 1:4 ish ratios from planetary.
Also I see what your saying and I kinda wanna make that in real life.

I’m a little unsure about the x drive speed stuff cause I be never made one or remembered the calculations.

nope. that’s where the differential comes in. look at kyle’s cads in the vex cad server

while my original comment was (mostly) a joke, i’m sure it’s possible to make a 2m diffy swerve if you tried hard enough. doubtful it’d be better than normal designs tho cuz of frictional losses, but theoretically you can travel any distance in less time than any tank drive with the same power input assuming frictional losses are negligible. once rubber bands get involved tho, the physics all goes out the window. i dont like entropic elasticity
: (

Yea I know I’ve dabbled in the dark art of diffrentials before. Also should be considerably more compact than what he made. All you would need are bevel gears connected to a wheel.

1 Like

not exactly… you want to have independent control over the wheel’s rotation about itself and it’s rotation about the axis- that’s the whole point of the differential in this scenario

I have a similar thing with LEGO and it would work for this. It’s just bevel gears. (Arranged in a specific way)
1 Like

now try and do it with vex parts, and you end up with what kyle made


Technically you only need one wheel. You just need something like training wheels for a bike or some other device to prevent it from falling over.

1 Like